AM-FM radio receivers have benefited greatly from the use of integrated circuit (IC) technology. Performance has been improved and size greatly reduced. Most of the active devices can be incorporated into one or two IC chips that require a minimum of off-chip components. Thus, very small, compact high-performance FM-AM radios can be constructed at low cost.
In order to take advantage of the high-fidelity (Hi-Fi) capability of the FM band the audio amplifier section is usually made to have a relatively wide bandwidth, typically about 20 kHz. Such an amplifier using conventional design will have appreciable gain out to several MHz and will produce substantial energy in the AM broadcast band. Thus, in the AM mode of reception the audio signal will reproduce the modulation, and unfortunately, the signal transients will result in unheard and relatively useless signal components within the radio frequency passband. Also any noise generated in the amplifier will have substantial gain. Where the radio speaker and/or its associated wires are located in close proximity with the radio antenna, as would be the case in a compact radio structure, the transfer of signal energy can be appreciable. Such signals are interpreted by the receiver as noise and the normal reaction is for the receiver AGC circuits to reduce the receiver gain. This has the effect of reduced sensitivity. As a practical matter it has been found that such audio circuit noise can seriously impair AM radio performance.
It has become common practice to shunt the input of the audio amplifier with a capacitor in the AM radio mode so as to reduce the high frequency energy. While this reduces the amplifier fidelity, the loss may not be regarded as harmful because of the lower fidelity performance normally expected in AM broadcasting. However, the capacitor shunting requires an addition to the switching system that is undesirable.
In the copending application of Don R. Sauer and William A. Cocke, Ser. No. 947,981 filed Oct. 2, 1978, another solution to the problem is presented. The AM-FM radio is switched electronically by on-chip IC elements. At the same time AF amplifier bandwidth is switched so that a reduced audio fidelity is present for AM operation. This approach is useful in IC radio designs where a minimum number of chips are to be used.
It would be desirable to have an audio amplifier available which could be used directly without extra switching for radios whether AM, FM, or both. Desirably such an amplifier would produce Hi-Fi response while producing negligible signal output in the AM broadcast band.